In a semiconductor module such as a power semiconductor module, a semiconductor element is mounted on an insulating substrate. For the purpose of decreasing the increased temperature of the semiconductor element occurred during the operation of the semiconductor element, the insulating substrate is arranged on and joined to a cooling surface of a cooler (including a heat dissipation/radiation device, such as, e.g., a heat sink, a heat dissipation/radiation plate, etc.).
Such an insulating substrate includes a ceramic layer as an electrical insulating layer, a wiring layer (circuit layer), etc. (see, e.g., Japanese Unexamined Laid-open Patent Application Publication (hereinafter simply referred to as “JPA”) 2004-328012, JPA 2004-235503, JPA 2006-303346, JPA 2009-147123). The wiring layer is made of, e.g., copper, aluminum, etc.
As a cooler, a cooler made of copper or aluminum (including its alloy) is used. In recent years, a cooler made of aluminum is widely used. The main reason is that an aluminum cooler is light in weight and can be obtained at low cost.